In many applications such as in airborne electrical systems, size and weight of dynamoelectric machines such as electrical generators are of considerable concern. If the machine is undesirably large, it may prevent the use of a most desirable aerodynamic configuration for an engine nacelle with the result that an aerodynamic design with greater than desired drag must be used. As the weight of the machine increases, all other things being equal, the useful load of the aircraft is reduced. In either case, the aircraft cannot be operated as efficiently as would be the case if the machine was of small size and relatively low weight.
In order to minimize size and weight of aircraft generators, the number of poles in the generator have been reduced. Consequently, there are presently aircraft generators that operate at 24,000 rpm necessary with a two-pole generator to generate the desired 400 Hz alternating current power universally employed on all but the smallest aircraft.
As is well-known, centrifugal force in a rotating body is proportional to the angular velocity or rate of rotation of the same. Thus, it will be readily appreciated that high centrifugal forces result during the operation of such high speed generators and it is necessary to provide a means of containment of rotor components, most notably windings, during generator operation. A common means of providing the desired containment is through the use of a containment sleeve surrounding the rotor periphery and fronting on the air gap between the rotor and the stator.
In order to minimize interference with the magnetic fields present during generator operation that would reduce generator efficiency, it is necessary that the sleeve be of a non-magnetic material. At the same time, the sleeve should have minimal thickness since it appears to the magnetic flux path to be part of the air gap between the rotor and the stator; and the greater the air gap, the greater the reduction in generator efficiency.
In addition to operating at high rotational speeds, such generators are designed to operate at what might be termed a high power density, which is to say that large electrical currents are flowing in closely grouped conductors. As a consequence, substantial heat is present and such generators typically provide some sort of means for flowing a coolant through the rotor in heat exchange relation with the windings to prevent overheating to the point where insulation between the windings would be destroyed or otherwise damaged. While in some cases special conduits may be formed in the rotor core itself to conduct the coolant, typically oil, this may be undesirable for any of a variety of reasons. For one, the conduits may interfere with magnetic flux paths and either reduce generator efficiency or necessitate a somewhat larger rotor in order to achieve the desired flux path. Neither is acceptable and as a consequence, and as more fully described in commonly assigned, co-pending U.S. Pat. No. 4,647,804 issued Mar. 3, 1987 to Wefel, it is advantageous to employ windings made of round wires so that axially elongated interstices between adjacent winding exist along the length of the rotor. The coolant may then be flowed through such interstices in intimate contact with the windings to provide excellent cooling. Such coolant flow paths do not interfere with magnetic flux pattern, nor do they require special fabrication techniques. At the same time, because the rotor core typically will be made up of a series of laminations, and in view of the high centrifugal forces present during operation, the coolant will have a tendency to weep radially outwardly through the interfaces of adjacent laminations. If the containment sleeve is not such as to contain the weeping coolant, it will find its way to the air gap and create highly undesirable so-called "windage losses".
It will also be appreciated that when such generators are quiescent, or even operating under substantially no load, they may be at ambient temperatures which, in the case of aircraft, can be quite low. At the same time, under high loading conditions, where oil is used as a coolant, they may operate at much higher temperatures so long as such temperatures are somewhat short of the boiling point of the coolant.
Unlike generators at fixed power stations, aircraft generators are cycled between off or quiescent, non-operating conditions and operating conditions at substantial loads. The result is frequent thermal cycling of the generator components that are subjected to high heat as a result of the electrical currents flowing during operation. This can result in relative movement between the rotor and the containment sleeve and/or the generation of substantial thermal stresses and the ultimate failure of a containment sleeve.
The present invention is directed to overcoming one or more of the above problems.